Navigation and landing of aircraft in fog



Jun ma @43 E. F. W. ALEXANDERSQN 2,245,246

NAVIGATIDN AND LANDING OF AIRCRAFT IN FOG Filed Aug. 24, 1958 4 Sheets-Sheet 2 inventor: Ernst FW. Alexanderson,

by Wan/K His At orney.

x nl s N E /W Jun@ 10, 1941 E. F. w. ALEXANDERSQN 2245,24@

NAVIGATION AND LANDING OF !\.I.2CRFTv IN FOG Fied Aug. 24, 1958 4 Shee'ts--SheewI 3 45 Fig, 6.

/fece z; ver

mg, L

nvehtor His Attorney.

NAVIGATION AND LANDING OF IRCRAFT IN FOG 4 Sheets-Sheet 4 Filed Aug, 24, 195B lrwentorr Emst F Aexandeon,

wma, GMA

HIS Attorney.

I u M n 'u Patented .lune 10, 194i UNE? NAVGTEGN AND LANDING 0F AHRCRAFT 1N FOG ration of New York Application August Za, 1933, Serial No. 226,594

(El. 25i-li) i9 Claims.

My invention relates to radio systems for guiding aircraft in fog or under other conditions of low visibility, and particularly to radio systems for guiding the aircraft in fog over a predetermined course or trail in cross-country iiying and for landing the aircraft in fog at landm ing fields provided at intervals along the course. The invention has for its principal object to provide improved means for guiding an aircraft over an extended cross-country radio trail and for guiding the aircraft along predetermined glide paths to the landing iields.

In providing a system, for guiding aviator who is navigating in fog it is necessary that he be given at all times accurate information where he is and at what altitude above ground, From a practical point of View it is also important that the information be given him by a single simple radio apparatus on the aircraft and be -given in such form thai'. be can make use of it through a natural and instinctive mental reaction without need of interpretation and calcula tion. When dying in fog across country on. the radio trail and between the landing stations, the aviator requires information obtained from course signal means which will enable him to follow the trail7 information obtained from altitude signal moans which will enable him to ily at a predetermined altitude with reference to ground. The radio trail or course on which the aviator nies across country should terminate, at predetermined points along the trail. in radio systems for landing the aircraft. reduirem ments for a successful landing system are that a glide oath should be established lead" into the landing iield from the trail7 and tn .t the aviator should have direct and preferably visual indication whether he is to the right or left of this glide path whether he is above or below it In accordance with the present invention a novel, improved, and efficient system` .for guiding an aircraft over an extended cross-country radio trail or course in 'fog and for landing the aircraft in fog is provided.

The system comprises a series of trail transmitter stations spaced at relatively wide intervals along the predetermined cr .-country flying course or trail and arranged to transmit course radio signals and altitude radio signals, and similar transmitter stations, but spaced at relatively short intervals, located at the landing fields and arranged to transmit course signals and altitude signals along the predetermined glide paths leading to the landing hic-lds from the cross-country course or trail.

The course signal antennas at each station both along the cross country trail and at the glide paths are preferably so arranged as to pro vide at each station two beams directed horizontally with vertical polarization, the beams be* ing directed respectively a predetermined number of degrees to the right and left of the course. A dot-dash method of keying the course signal antennas is preferably employed.

The altitude signal is radiated at each station by two antennas spaced apart in the direction of the course and each providing a radiation directed upwards with a sharp directivity in the direction of the course but spreading fan-shape aloner the course. The two altitude beams at each station create an interference pattern in space so that the signal periodically disappears when the plane iiies through the pattern.

The system in accordance with my present invention provides, on the aircraft, a vcourse and altitude signal receiving means which preferably comprises a single receiver apparatus. For the course signal indication the receiver apparatus preferably includes a visual course indicator of the dot-dash type, wherein the instrument needle goes to the right or to the left if the aircraft is oi course but remains on neutral when the aircraft is on course. .For the altitude signal indication the receiver apparatus includes means, preferably providing a visual indication, responsive to the pulses created in the radio receiver by the interference pattern which is set up b-y the altitude signal transmitter. The altitude indicating means is so arranged that in flying the aircraft through an interference pattern of a given form, for a predetermined ground speed the instrument indicates directly in response to the pulsation frequency whether the aircraft was at the correct altitude it through this interference pattern.

The time of iiying through the interference natterns is so short particularly at relatively low altitudes that indication of aircraft altitude based on direct observation of the pulsation frequency is not usually feasible. An altitude indicating means is` therefore. provided which registers each nulsation separately and integrates or sums up the effect of the whole number of pulsations so that the pilot obtains the nal result when he has passed through the pattern. The indicati-ng instrument associated with this integrating means is so arranged that at the end of the passage of the aircraft through an interference pattern, the pointer comes to rest at a point, on the dial, indicating altitude and remains at this point until the instrument is reset to the olf point by the operator, ready for altitude indication at the next succeeding interference pattern.

Along the sections of the radio trail or crosscountry course, between the landing elds, the altitude signal transmitters are preferably so arranged that the interference patterns are alike throughout the sections.

At the glide path leading to a landing i'leld the several altitude signal transmitters may be so arranged that the interference patterns are all different. The several interference patterns may be, for example, so adjusted and the several beacons or stations maybe so spaced along the glide path that at the .predetermined gradually decreasing speeds of the aircraft as it cornes down the glide path toward the ground, the successive maxi-ma of radiation, in the succession interference patterns along the glide path are passed by the aircraft at constant intervals of time and the frequency lof the pulses received on the aircraft and due to the passing of the aircraft successively through the patterns is, therefore, the same at all of the `patterns. In this case the pointer of the altitude indicating instrument on the aircraft comes opposite a certain point on the dial, for example the middle point, if the plane passes the rst landing beacon at a predetermined glide path altitude and at the correct predetermined speed at that beacon. The pointer then comes to this same predetermined point for the successively lower altitudes at the succeeding beacons if the plane passes at the predetermined altitudes and speeds for these beacons.

The several altitude signal transmitters at glide path may, however, be so arranged that the interference patterns are all alike, similarly, for

example, to the interference patterns along the cross-country radio trail sections. In this case, unless the ratio between the predetermined dcsired speeds along the glide path throughout the series of stations is inversely as the ratio between 'hc desired glide path altitudes throughout the series of stations, the pulsation frequency received by the aircraft will not remain the same, as the aircraft passes the series of stations. In practice, the speeds and altitudes will usually be such as the aircraft passes the stations that the pulsation frequency will be least at the rst station and will progressively increase to the last station. In this latter case, since the received pulsaticn frequency is changing throughout thc passage of the aircraft down the glide path, adjustment of the indicating instrument will be required at each succeeding station of the glide path to compensate for the increasing frequency, if it is desired that the pointer corne to the same point on 'the dial to indicate the correct glide path altitude of the aircraft for each station on the glide path.

Instead of indicatin f the correct glide path altitude at each station by merely bringing the pointer to the one point on the dial as above lmentioned, the indicating instrument may be arranged to show, on a single scale, the actual altiu tilde of the aircraft above ground in meters or iect at each succeeding station along the glide path, adjustment of the instrument being required as before at each succeeding station to compensate for the increasing frequency. If, however, it is desired to avoid the necessity of adjusting the instrument, as the aircraft passes the succeeding stations, to compensate for the increasing frequency, the instrument may be provided with a multiple scale, each scale showing directly and without further adjustment of the instrument a range of altitudes for each glide path station, provided that the speed in passing a station is the aircraft speed desired for the portion of the glide path at that station.

Since in any case the correct indication of aircraft altitude -at the successive stations along the radio trail is dependent upon the speed of the air-craft as it passes the stations, and at the glide paths is dependent upon the gradually decreasing speeds at which the aircraft passes through the succeeding interference patterns, a means is provided to adjust the altitude indicating instrument to compensate for speed vari 'ion from the predetermined rates of speed at .e Various stations.

The novel features which are considered to be characteristic of my invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation together with further obiects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawings, wherein Fig. 1 is a side elevational diagrammatic view showing a series of course and altitude radio stations for guiding an aircraft in fog, and illustrating diagrammatically interference patterns produced by the altitude signal transmitters; Fig. la shows a modification in which the interference patterns are so arranged that at predetermined decreasing speeds of the plane down the glide path the frequency of the received pulses is the same at all of the patterns; Fig. 2 is a plan view of the series of course and altitude signal stations illustrated in Fig. 1 but showing the diagrammatically the course signal radiation patterns produced by the course signal transmitters; Fig. 3 is a circuit diagram of a course and altitude signal transmitting apparatus; Fig. i illustrates a coupling means to prevent interaction between the course signal an. altitude signal circuits of Fig. 3; Fig. 5 shows in detail the course signal transmitting antenna units of Fig. 3; and Fig. 6 is a circuit diagram of a course and altitude signal receiving apparatus.

In Fig. 1 the numeral lil designates ground level, and the numerals il, l2, i3 and ifi designate respectively course and signal transmitting stations each including a course signal transmitting antenna unit l5 and a pair of altitud-e signal transmitting antennas i6. The numeral il indicates the predetermined glide path down which it is desired that an aircraft E8 pass in making a landing. The glide path is a continuation cf a cross-country aircraft course or aircraft radio trail (not shown) over which the aircraft is guided in fog by the signals from a series of course and altitude signal transmitting stations (not shown) similar in general to,stations l l to ill but spaced at relatively long intervals. The

frequency of the course and altitude signals is lesignate respectively interference patterns which nay be produced by stations II, I2, I3 and I.

In Fig. 2 the numerals 23 and 24 designa-te airs of antennas constituting the course signal ransmitting antenna units I5 which are aranged to radiate pairs vof beams 25 and 26 di ected horizontally with vertical polarization. .The antennas 23 and 24 are preferably so aranged that .the beams 25 and 26 are radiated espectively to the right and left of the course y such an amount that when the avia/tor is fly- .ig to one side of the course he will be within he beams of both antennas of the unit but the ignal from the beam on the side where he is lying will still be of suificiently greater strength han the signal from the beam on the other side o insure an adequate off-course indication to the via-tor, by the dot-dash or other method. Prefrably the angular arrangement of the antennas 3 and 2li should be such that one of the beams 25 i directed 1'7 degrees to the right of the course nd the other beam 26, is directed 1'? degrees to ylie left of the course.

In Fig. 3, the course and altitude signal transiitting apparatus comprises a radio transmitter l for producing oscillations of suitable power od frequency. The output circuit 2'8 of transritter 2l may be connected directly to the two ltitude signal. antennas I S through leads 29, 3B ud 3I. Each altitude signal antenna Iii is prefably of the panel type, provided with a reector (not shown) in order that the radiation 'om these antennas may be unidirectional. he conductors of the Ipanel are in the direcon of the aircraft course or trail and each panel insists :of a suitable number of half-wave radi- ;or units and the same numb-er of reiiector nits.

`The radiation from the course signal antenna lit I5 is controlled by means of a relay aptratus comprising a rel-ay 32 through the cone lcts of which 'the output circuit 28 is adapted to i connected to the antenna 23 and 24. The con* .cts of relay 32 are actuated by lan operating coil i energized from a current sourcev 34 controlled 1 a commutator 35. The c-ommutator 35 and rey 33 are so arranged 'that the antenna unit I5 adapted to be keyed by the dot-dash method, itenna 23 radiating dots and antenna 2li radiing dashes when the cOmIn-utator 35 is rotated id oscillations are supplied to the antenna unit om transmitter 27. The antennas 23 and 2li of the course signal rtenna unit I5 are preferably of a simple zigzag pe which gives the desired radiation for the urse signal. In 3 the antennas 23 and 2i-i e 'shown rotated 90 degrees from their true po- ;ion with reference to the ground. It is preferable to connect the transmitter 2? the antennas i5 through the coupling means illustrated in Fig. 4. in order to prevent intertion between the course signal and altitude sig il transmitting systems. This coupling means mprises a transformer 3'! having a primary 38 nnected to the transmitter and a secondary 39 'justably connected to the leads 35i and 3l, and capacitance iii connected across the secondary. Antennas and 24 are better shown in 5 eroin the unit l5 is illustrated as mounted adstably on a mast 4I by links or hinge-s 52 per- .tting the swinging of the antennas 23 and 2li th in the vertical and horizontal direction with ference to the mast. To provide uni-direcnal radiation of the course signals, the an- 1 Util 4tenrras 23 and 2li are provided respectively with reflectors 43 and lili. In order that the course signal radiation may be of the hereinbefore described desired pattern the two antennas 23 and 24 are respectively oriented horizontally on the opposite sides of Ithe course, approximately 17 degrees to a line perpendicular to the course directio-n,

In Fig. 6 the course and altitude receiving apparatus is shown. This equipment comprises an antenna 45 yof any suitable type but which ls preferably of the loop type oriented in Ithe length direction of the aircraft so ythat the antenna receives the course signal when the aircraft is approaching one of the trail or landing beacons or stations or is flying between these stations, and also can receive tl'ie altitude signal-s coming from below when the aircraft is passing over the stations. Tlie receiving apparatus further comprises a radio receiver "iii, of any well known or preferred type, for amplifying and detecting the dotdash-modulated waves from the course signal transmitters, and for amplifying the periodically uctuating waves from the altitude signal transmitters. The output circuit 4'! is coupled to the input circuit d3 of an amplifier 49 the output -circuit of which includes in series a visual course indicator 58 of the dot-dash type which responds to dot-dash course signals in a manner t-o be described hereinafter. Anode potential for the amplifier t9 is supplied from a potentiometer 5I connected across a direct current source.

To provide visual indication -of altitude, the receiving apparatus comprises an altitude instrument 52 which is adapted -to be actuated, at each passing of the aircraft through an interference pattern, in accordance with the amount of charge in a condenser 53. This condenser registers the peak charge on a condenser 54 which in turn is charged in accordance with the pulsation freouency oi the altitude signal as the aircraft passes through an interference pattern. For this purpose the receiving appara-tus comprises a discharge device or tube 55, having an output circuit including the above mentioned condenser 54 and a leak resistor 56 in parallel therewith, potentiometer 5i, and an anode resistor 51, and having a control electrode '58 normally biased negatively by connection of the control electrode 53 and vthe cathode 59, through a resistor iiD, across va portion of a potentiometer 6I connected across a direct current sour-ce.

To impress charges on condenser Eli in accordance with the frequency of the pulsations due to passing of the aircraft through an interference pattern, a relay 52 is provided having its actuating winding S3 connected Iin series in the anode circuit of ampliiier tube dii. The relay 52 has two sets of contacts iii! and B5. When the contacts lili are closed, by energizing of the coil B3 du@ to the reception by antenna 45 of a strong current nulse due to the transmitted altitude` signal, control electrode 58 is short circuited momentarily to cathode 53 by a condenser '643 con nected between this cathode and the contacts S4. The potential on contr-ol electrode 5i) then rises to a value approaching the cathode potential. When the contracts P35 are in closed position, as illus-trated, due to the condenser Se?. and this latter condenser is connected across a discharge resistor 61 and discharged therethrough. At the instant that the Home! passing of the antenna l5 through a zero voltage section of the interferencel pattern, control electrode 581s disconnected from Efection with tlie"navigation scale, and the landing scale is suitably calibrated for the low altitudes.

As hereinbefore mentioned, the altitude signal transmitters along the glide path at a landing field may be so spaced and may set up interference patterns of such form that, at the decreasing predetermined speeds of the aircraft through the successive interference patterns, the pulsation frequency at the glide path will not be the same at the successive stations, being, for example, the least at the first beacon and progressively increasing to the last. If a single scale is employed in instrument 52 to indicate the altitude down such a glide path, compensation for the changing pulse rate is required, and is obtained, in the manner above explained, by suitable adjustment of. the resistor G which regulates the rate of leakage of the charging current for condenser 5d.

In describing the operation of the present invention in guiding an aircraft through fog over a cross-country radio trail and down glide paths to landing fields located along the radio trail, let it be assumed that the pilot, flying on the radio trail at an altitude, for example, 01; 5,000 feet above ground, encounters iog and thereupon prepares 1`or the guiding of the craft by the radio signals from the radio trail or course beacons or stations. fhe cross-country 'radio trail stations may be spaced about ten miles apart and all or most of them may be located at points where an aircraft iight beacon already exists, although the light signals are not visible in i'og. The pilots first concern is to follow this cross-country radio trail, which may, for example, be a Winding path through mountain passes. For this purpose he tunes in his radio receiver it on the frequency oi' the course and altitude signal transmitters and observes the visual course indicator Sil. Tube fia oi' receiver #iii is so biased by connection or its control electrode to potentiometer 5l that in the f.,

absence of the dot-dash modulation of the course signal the impressing of this signal on the tube would result in a non-pulsating output current whose amplitude, indicated by the moving of the pointer of instrument 5t toward the right of the dial, increases as the distance of the plane from the transmitting station which the plane is approaching decreases. When, however, the course signal is dot-dash modulated as hereinbeiore described, assuming, iirst, that the plane is directly on the proper course, the dot signals radiated by antenna 23 and dash signals radiated by antenna lili are equal in magnitude at a given time period. Therefore, under these on-course conditions, since the signals impressed by the two antennas 23 and 24 on tube 9 are of equal magnitude, the output current of tube i9 is practically steady, or non-puisating, and the pointer of instrument 5d merely moves steadily to the right as the distance between the plane and the station decreases.

Assuming, second, that the plane is, at a given time, to the right of the proper course, as at point p1 of Fig. 2, then the strength of the signal received from antenna 2li will be greater than the signal strength for the ori-course condition and the strength of signal received from antenna 23 will be less than for the ori-course condition. Therefore, at this given time the position of instrument 5b indicates a relatively steady current, slightly higher than for the on-course condition, due to the dash signals from antenna 213, and the pointer periodically shifts or kicks to the left to indicate the periodically recurring lower signal current pulses due to the dot signals from antenna 23.

Assuming, third, that the plane is, at a given time, to the left of the proper course, as at point p2. In this latter case the strength of the signal received from antenna 23 will be greater and from antenna Ml less than for the on-course signal. Therefore, the pointer of instrument indicates in this third case a relatively steady current, due to the dash signals from antenna which is slightly lower than for the oli-course condition, and the pointer periodically shifts or kick to the right to indicate the periodically recurring higher signal current pulses due to the dot signals from antenna 23.

rf'herefore, if the pilot is to the right of the proper trail, the pointer of the course indicator til will maire periodic motions or kicks to the leit. Il he is to the left of the proper trail the pointer of indicator 5d will make periodic motions or kicks in the other direction. By correcting his course in accordance with the readings o the course indicator 5&3 he will bring the plane directly on the trail, in which position the course indicator pointer remains substantially steady, the pointer merely moving across the dial to indicate change in distance between the plane and the transmitting station which the plane is appreaching.

As the pilot then approaches this beacon or station on the radio trail, the received signal l'rom the course signal antenna will become progressively stronger, this increase of course signal being indicated as above explained by the steady movement oi' the pointer of course indicating instrument 5&3 to the right.

The trail as indicated by the pilots chart may change direction at this next station and it is, therefore, desirable that he Should be able to identify this station. lie may conveniently receive this information While he is approaching and before he arrives at the station, 'by a code signal which is given him from time to time. This code signal may be supplied from the course signal transmitting system by any suitable lreying means, be associated with the transmitter 2i, and the code signal may be observed on the course indicator iid.

Before entering the interference pattern set up by the altitude radiation from the altitude transmitter unit oi the radio trail station which he is now approaching as shown by the stronger signal indications in course indicator til, the pilot will adjust the altitude indicating system, at resistor 5t, for the use of the navigation scale of altitude indicating instrument 52. Just before entering the interference pattern be maires, at resistor d5, an adjustment of the altitude indicating system ii necessary, to compensate for a speed of the aircraft, as indicated by the air speed indicator, which may be different from the predetermined trail speed, in order that thealtitude indicating system may be in readiness to indicate altitude more nearly correctly. He also opens ie normally closed short circuiting switch i5 for Condenser 53 in order that altitude indicator 52 may he in readiness to function. As the aircraft now passes through the interference pattern set up by the two altitude signal transmitters at the station, the pointer i3 of altitude instrument 52 moves downward from its extreme upper position iirst fast and then slowly, and nally stops. rThe pointer "i3 remains in its lowest position, as the aircraft leaves the interference pattern. Altitude is thus indicated to the ptilot by the point of the dial at which pointer 73 s ops.

i l u y In addition to making an adjustmentof the altitude indicating` apparatus in accordance with the aircraft speed as indicated by the air speed indicator, the pilot should, furthermore, make an adjustment of the altitude indicating apparatus to compensate for the wind velocity, since the altitude instrument responds to the pulsation frequency due to the passing of the aircraft through the fixed interference pattern, therefore to the ground speed of the aircrai". il the pilot does not know the wind velocity he can, however, utilize the altitude indicating system on the aircraft to ascertain 'the velocity. For this purpose he flics over the train station first in one direction and then in the opposite direction. at the saine barcrnetric altitude. If, for example, he measures 5000 feet in one direction and 6000 feet in the other direction he concludes that the true altitude was 5500 feet and that the wind 'velocity was l per cent of his 'flying The altitude indicating system having been adjusted for speed Variation and for wind velocity, in the trail direction as above described, correction of the altitude of the aircraft can then be made if necessary as the craft proceeds toward the succeeeding station. After the altitude at the station just passed through has been noted and altitude correction made if required, the short circuiting switch for condenser 53 is again closed to reset altitude indicator' 52 to the ofi point, and the pilot continues on the radio trail, guided by the indications of the course indicator 50. The altitude of the aircraft is determined as above described, and corrected if necessary, at each of the succeeding stations passed through by the aircraft in flying crosscountry along the radio trail.

Thus it will be seen that the pilot is able to navigate the aircraft on the cross-country radio trail with accuracy and with confidence, without seeing the ground, and without it being necessary for him to reduce his altitude above ground until he approaches a landing iield.

Assuming that it is his intention to land the aircraft at the field, he begins at a suitable point along the trail to descend gradually from the trail altitude, assumed to be G00 ft., to a lower altitude, for example i000 ft., and while doing so he changes the adjustment of the altitude indicating system from the scale labeled navigation to the scale labeled landing Instead of so arranging the transmitter stations that the lpilot begins to descend the landing station glide path as soon as the aircraft has reached the 1000 foot level after descending from the 5000 foot level, it is preferable that the craft first pass over, at the 1000 foot altitude, a series of course and altitude Signal stations or beacons which may be called landing beacons. These are of the same type as the cross-country radio trail stations or beacons but are spaced for some distance at substantially shorter intervals than the cross-country trail beacons. The pilot veries and corrects, if necessary, his course and altitude as he passes these landing beacons. When he arrives within a suitable distance, for example two miles, of the landing field, he starts his landing glide down the glide path.

A suitable number of beacons may be located along the glide path, for example four beacons corresponding respectively to glide path altitudes from 1000 feet at the first down to 200 feet at the last of the series. Let it be assumed as hereinbefore mentioned that the glide path beacons are so spaced, for example 2000 'feet apart, and

ascenso the several interference patterns from the first to the last along the glide path are of such dif- Jferent form as illustrated in general, for example in Fig. lc, that at the correct predc'crniined decreasing speeds of the aircraft down the glide path the successive maxima of radiation from the glide path altitude signal transmitters are passed by the aircraft at constant intervals of time. ln Fig. la, assuming that it is desired that the airplane speed decrease from a predeterrelatively high value at pattern i9 through predetermined loi-.vcr values at patterns 20 and to a predetermined value, at pattern 22, relatively low as compared with the high value at pattern i0, then the patterns are so spaced (equally in the present case), and are respectively of such i'orrn, that the intervals between the maxima of radiation along the glide path increase in length approxin'iately directly as the predetermined speeds from the pattern i9 to 'the pattern 22. In this case if, upon descending past the several beacons of the glide path, the pilot notes that the pointer of the altitude indicator 52 has stopped each time at the same predetermined point of the dial, for example the middle point, 'ne then knows that the aircraft was at the predetermined correct altitudes for the several beacons, provided that the speed of the aircraft was the correct predetermined speed at each beacon. Il he iinds that the speed is not correct before entering any one of the several interference patterns, the pilot makes compensating adjustment at resistor 5t, as hereinbefore explained. Ii' the instrument indicates, after the aircraft passes a glide path beacon, that the altitude was not correct for that beacon, the pilot changes his rate of descent correspondingly before the aircraft enters the interference pattern of the next succeeeding glide path beacon. After passing a beacon of the glide path, and verifying the altitude, the pilot returns the altitude indicator 52 to the ofi point by closing switch i5, and in the intervals between beacons he corrects his course, if necessary, by the indications of the course indicator dii. After leaving the last glide path beacon of the series, the aircraft glides to a landing.

In case the altitude signal transmitters along the glide path are so arranged that the interference patterns are all alike, the operation of gliding down the glide path to a landing is the same as just described except that, whether the altitude indicating apparatus be arranged to bring the pointez' of indicator 52 to one predetermined point for each beacon as in the case just described, or on the other hand be arranged to show on one scale the altitude at each beacon in meters or feet, compensation for the increasing rate of pulsation of the charging current into condenser li during the descent of the aircraft down the glide path is required as the aircraft enters each one of the series of interference patterns. if, however, the hereinbefore mentioned multiple scale dial for altitude indicator 52 is employed, having a plurality of scales calibrated respectively to indicate altitude in meters or feet at the successive beacons of the glide path, the operation of gliding down the glide path to a landing will be as above described for the single scale indicator system showing actual altitude in meters or feet, except that in the case of the multiple scale system, no compensation for the increasing pulse rate of the charging current for condenser 513 is made by the pilot.

My invention has been described herein in particular embodiments for purposes of illustration. It is to be understood, however, that the invention is susceptible of various changes and modifications and that by the appended claims I intend to cover any such modifications as fall Within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States, is:

1. In a system for guiding an aircraft along a predetermined course comprising a transmitter apparatus located on said course including means for producing altitude signal radiation and for producing in said radiation an interference pattern which is fan-shaped in the direction of said course, a receiving system arranged on the craft to receive impulses due to the successive radiation maxima in said interference patterns as the craft traverses said radiation, and means to integrate said impulses and to indicate altitude of said craft from said integrated impulses comprising .a condenser, means to charge -said condenser to a peak potential corresponding to the frequency of said radiation maxima received by said aircraft at a given altitude thereof, a second condenser, means to charge said second named condenser from said first-named condenser to said peak potential, means to maintain said second-named condenser at said peak potential, and means for indicating the altitude of said aircraft in accordance with the charge on said second-named condenser.

2. In a system for guiding an aircraft comprising a transmitter apparatus for producing altitude signal radiation and for producing an interference pattern therein which is fan-shaped longitudinally of the predetermined course of said aircraft, signal impulse integrating and altitude indicating means on said aircraft responsive to said radiation comprising a condenser, means to charge said condenser to a peak pou tential corresponding to the frequency of said radiation received by said aircraft at a given altitude thereof, a second condenser, means to charge said second-named condenser from said first-named condenser to said peak potential, means to maintain said second-named condenser' at said peak potential, and means to indicate the altitude of said aircraft in accordance with the charge on said second-named condenser.

3. In a system for guiding an aircraft along a course comprising signal transmitting means on said course, signal receiving means On said aircraft, said receiving means being adapted to receive from said signal transmitting means periodic potentials varying in frequency inversely as the altitude of said aircraft, means connected with said receiving means to integrate said periodic potentials and to indicate altitudes of said craft from said integrated potentials comprising a condenser, means to charge said condenser to a peak potential corresponding to the frequency of said potentials received by said aircraft at a given altitude thereof, a second condenser, means to charge said second-named condenser from said first-named condenser to said peak potential, means to maintain said second-named condenser at said peak potential, and means to indicate the altitude of said aircraft in accordance With the charge on said second-named condenser.

4. The combination, in a system for guiding an aircraft along a course at a predetermined speed, of a transmitting apparatus on said course for producing altitude signal radiation and for producing an interference pattern therein which is fan-shaped longitudinally of said course, and means for indicating altitude of said aircraft as said aircraft passes through said Pattern at a predetermined speed, said means comprising a condenser, means to charge said condenser to a peak potential corresponding to the frequency of the radiation maxima of said pattern at the altitude level thereof at which said aircraft passes therethrough at said predetermined speed, a second condenser, means to charge said secondnamed condenser to said peak potential from said first-named condenser, means to maintain said second-named -condenser at said peak potential, and means to indicate the altitude of said aircraft in accordance With the charge on said second-named condenser.

5. The combination, in a system for guiding an aircraft along a course at a predetermined speed, of a transmitting apparatus on said course for producing altitude signal radiation and for producing an interference pattern therein which is fan-shaped longitudinally on said course, and means to indicate for a 4desired period after passage of said aircraft through said pattern the altitude at which said aircraft passed therethrough, said means including a condenser adapted to receive charging current only during said passage of said aircraft .and to be charged to a peak potential corresponding to the frequency of the radiation maxima of said pattern, a second condenser adapted to be charged from said first-named condenser to said peak potential during said passage, and means connected in series with said condensers to prevent leakage of the charge on said second condenser after said passage whereby said second 'condenser remains operative for indicating altitude after said passage of said aircraft through said pattern.

In a system for guiding an aircraft down a predetermined glide path to a landing field, a plurality of transmitter stations spaced at predetermined intervals below said glide path and in the vertical plane thereof, means at each station for producing a pair of course signal beams radiated respectively a predetermined number of degrees to right and left of said vertical plane, means at each station for producing altitude signal radiation directive in the direction of said vertical plane and for producing a predetermined interference pattern in said radiation, a single receiver unit on said aircraft, means in said unit to indicate the altitude of said aircraft responsive to periodic potentials impressed upon said unit due to the passing of said aircraft through said pattern, means in said uni-t responsive to said course signal beams to indicate the course of said aircraft and to indicate the approach thereof to a succeeding one of said transmitter stations, and means in said unit 'to transmit said periodic potentials through said course signal indicating means.

7. In a system for guiding an aircraft from a point on a cross-country trail down a glide path to a landing field, a plurality of transmitter stations spaced at relatively vide intervals along said trail and at relatively short intervals along said glide path, means at each station for producing a pair of course signal beams directed horizontally With vertical polarization and radiated respectively a predetermined number of degrees to right and left of the predetermined course of said aircraft, means at each station for producing altitude signal radiation highly directive .in the direction of said course and so arranged as to produce a predetermined interference pattern in said altitude signal radiation, a single receiver unit on said aircraft, means in said unit to indicate the altitude of said aircraft responsive to periodic potentials impressed thereon when said aircraft passes through said pattern, means in said unit responsive to said course signal beams to indicate the course of said aircai't and to indicate the approach ci' said aircraft to a succeeding one of said stations, and means in said unit to transmit said periodic potentials through said course signal indicating means.

8. In a system i'or guiding an aircraft from a point on a cross-country trail down a glide path to a landing eld, a plurality of transmitter stations spaced along said trail and said glide path, a course signal means at each station including a pair of antennas for producing a pair of beams directed horizontally with vertical polarization and radiated respectively substantially seventeen degrees to right and left ci' the predetermined course of said aircraft, means at each station including a pair of antennas spaced longitudinally of the predetermined course -oi' said aircraft for producing altitude signal radiation highly directive in the direction of said course and for producinga fan-shaped nterference pattern in said radiation, a single receiver unit on said aircraft, means in said unit to indicate the altitude of said aircraft responsive to periodic potentials impressed upon said unit when said aircraft passes through said pattern, means in said unit responsive to said course signal beams to indicate the course of said aircraft and to indicate the approach thereof to a succeeding one of said transmitter stations, and means in said unit to transmit said periodic potentials through said couse signal indicating means.

9. In a system for guiding an aircraft down a predetermined glide path to a landing field at a speed decreasing at a predetermined rate, a plurality of .altitude signal transmitting stations spaced at predetermined intervals along said glide path, pulse receiving means'on said aircraft, and a plurality of fan-shaped interference patterns produced by said stations respectively to cause pulses of approximately the same frequency to be impressed on said receiving means as said aircraft passes through all of said patterns along said predetermined glide path and at said speed decreasing at a predetermined rate.

l0. In combination, in a system for guiding an aircraft down a predetermined glide path to a landing field at a speed decreasing at a predetermined rate, a series of altitude signal transmitting stations spaced at predetermined intervals along said glide path for producing identical interi'erence patterns intersected by said glide path, altitude signal receiving means on said aircraft including altitude indicating means responsive to the successive radiation maxima in said patterns, and means to adjust said altitude indicating means to compensate for the Variation in frequency of said radiation maxima as said aircraft passes through said patterns.

11, In a system for guiding an aircraft along a course comprising signal transmitter means on said course, altitude signal receiving means on said aircraft including a condenser and a leal; resistor in parallel therewith, said signal transmitting means being adapted to impress on said receiving means periodic potentials varying in frequency inversely as the altitude of said aircraft, means for supplying current pulses to said conde.. ser in accordance with ,said potentials, a second condenser, means for charging said second condenser to the peak.' potential of said firstnamed condenser and for maintaining said sccond-named condenser for a desired period at said peak potential, and means for indicating altitude of said aircrart in accordance with the charge on said second-named condenser.

l2. In a system i'or guiding an aircraft along a course comprising signal transmitting means nn said course, altitude signal receiving means on said .aircraft including a condenser and a leak resistor in parallel therewith, said signal transmitting means being adapted "to impress on receiving means periodic potentials varying in frequency inversely as the altitude oi' said aircraft, means i'or supplying current pulses to said condenser in accordance with said potentisls, a second condenser, means for charging said second condenser to the peak potential of said tiret-named condenser and for maintaining said second-named condenser at said peak potential, means for indicating altitude of said aircraft in accordance with the charge on said second-named condenser, said last-named means including a spaccdischarge device having a control electrode adapted to have a potential impressed thereon from said second-named condenser, and means to discharge said secondnamed condenser and to impress a negative bias potential upon said control electrode.

13. 'in a system lor guiding an aircraft along a course comprising signal transmitting means on said course, altitude signal receiving means on said aircraft including a condenser and a leak resistor in `parallel therewtl'i, said signal transmitting means being adapted to impress on said receiving means periodic potentials varying in frequency inversely as the altitude of said aircrai t, means for supplying current pulses to said condenser in accordance with said potentials to charge condenser to peak potential corresponding to the frequency oi said periodic potentials received by said aircraft at a given altitude thereof, a rectifier, a second condenser, means for connecting said second condenser tlnougii said rectifier across said first-named condenser to receive a charge at said peak potential therefrom, and means for indicating altitude oi said aircraft in accordance with the charge on said second condenser.

le. 'Ihc combination, in a system for guiding an aircraft down a pre-deter' ned glide path to a landing field, of altitude s goal receiving means on said aircraft including a condenser and a leak resistor in parallel therewith, a series of sie, zal tr nsmitting stations spaced along said path for impressing on said receiving means pcriodic potentials said aircraft passes said stations successivcly, means for supplying current pulses o said condenser in accordance with said potentials, means for indicating altitude of said aircraft at each of said stations in accordance with the peak charge on said condenser, the leali resistance supplied across said condenser oy said resistor being adapted to 4be increased rogrcssively as said aircraft passes said stations successively, thereby to compensate said indicating means for said progressive increase in frequency oi' said potentials.

l5. The combination, in a system for guiding an aircraft along a course, of a single receiver unit on said aircraft for receiving course signals and altitude signals, a series of signal transmitting stations on said course each including 4transmitting means for impressing course sigu nais on said receiver unit When said aircraft is passing from one station to the neX-t in the series and altitude signal transmitting means for impressing periodic potentials on said receiver unit When said aircraft is Vapproaching and passing over any one of said stations, means in said receiver unit for indicating altitude of said aircraft in response to said periodic potentials, means in said receiver unit for indicating the course of said aircraft in response to said course signals, and means to transmit said periodic potentials through said course signal indicating means.

16. In a system for guiding an aircraft down a predetermined glide path to a landing field comprising yan altitude signal transmitting sta-A tion for producing a fan-shaped interference pattern intersected by said glide path, altitude signal receiving means on said aircraft including altitude indicating means responsive to the successive radiation maxima in said pattern and adapted to indicate a predetermined altitude of said aircraft at said station and at a predetermined speed of said aircraft when passing said station, said altitude indicating means comprising a condenser, means to charge said condenser to a peak potential corresponding to the frequency of said maxima received by said aircraft at a given altitude thereof, a second condenser, means to charge said second-named condenser to said peak potential from said first-named condenser, means to maintain said second-named condenser at said peak potential, means to indicate said altitude of said aircraft in accordance with the charge on said secondmamed condenser, and a resistor connected across said first-named condenser and adapted to be varied thereby to adjust said altitude indicating means to compensate for variation of the speed of said aircraft from said predetermined speed.

17. In a system for guiding an aircraft along a course comprising altitude signal transmitting means on said course, altitude signal receiving means on said aircraft including a condenser and a leak resistor in parallel therewith, said signal transmitting means being adapted toimpress on said receiving means periodic potentials varying in frequency inversely as the altitude of said aircraft, means for supplying current pulses to said condenser in accordance With said potentials, a second condenser, means for charging said second-named condenser from said firstnamed condenser to the peak potential of said first-named condenser, means comprising a reetier connected in series with said condenser to maintain said second-named condenser at said peak potential, and means for indicating altitude of said aircraft in accordance With the charge on said second-named condenser due to said current pulses.

18. In a system for guiding an aircraft along a course comprising a signal transmitting means on said course, altitude signal receiving means on said aircraft including a condenser and a leak resistor in parallel therewith, said signal transmitting means being adapted to impress on said receiving means periodic potentials varying in frequency inversely as the altitude of said aircraft, means for supplying current pulses to said condenser in accordance with said potentials, means for indicating altitude of said aircraft in accordance with the peak potential on said condenser, said indicating means comprising a second condenser, means to charge said secondn named condenser to said peak potential from said nist-named condenser, means to maintain said second-named condenser at said peak potential, and means to indicate altitude of said aircraft in accordance with the charge on said second-named condenser, the leak resistance provided by said resistor being adapted to be varied thereby to vary the time constant of the peak charge on said first-named condenser.

19. In a system for guiding an aircraft along a course comprising a signal transmitting sta*- tion on said course, altitude signal receiving means on said aircraft including a condenser and a leak resistor in parallel therewith, said signal transmitting means being adapted to impress on said receiving means periodic potentials varying in frequency inversely as the altitude of said aircraft and directly as the speed thereof in passing over said station, means for supplying current pulses to said condenser in accordance with said potentials, means for indieating altitude of said aircraft in accordance With the peak potential on said condenser due to said current pulses, said indicating means comprising a second condenser, means to charge said second-narned condenser to said peak potential from said irstna1ned condenser, means to maintain said second-named condenser at said peak potential, and means to indicate the altitude of said aircraft in .accordance with the charge on said second-named condenser, the leak resistance supplied across said iirst-named condenser by said resistor being adapted to be varied thereby to compensate said indicating means for variation of the speed of said aircraft from a predetermined speed as it passes over said station.

ERNST F. W. ALEXANDERSON. 

